Biodiesel has been the subject of much investigation as an alternative for petroleum diesel fuel. As used herein, the term “biodiesel” refers to an ester-based fuel oxygenate that is derived from a biological source. The biodiesel is used as an alternative for, or as an additive to, petroleum diesel fuel in automobiles or other vehicles. The biodiesel is typically produced from a triglyceride starting material or a fatty acid starting material by a transesterification reaction or an esterification reaction, respectively. Generally, the triglyceride is reacted, or transesterified, with an alcohol to produce glycerol (also known as glycerin) and a corresponding alkyl ester of the triglyceride. Similarly, the fatty acid is reacted, or esterified, with an alcohol to produce a corresponding alkyl ester of the fatty acid. Large amounts of the triglyceride and fatty acid starting materials are available from inexpensive sources, such as fats or oils. However, since these fats or oils are too viscous to use directly as the biodiesel fuel, the triglycerides or fatty acids are transesterified or esterified to produce the corresponding alkyl ester, which has a lower viscosity than that of the starting material. As such, the corresponding alkyl ester is able to be used as the biodiesel fuel.
The transesterification of the triglyceride (or the esterification of the fatty acid) is conducted with an excess of the alcohol in the presence of a catalyst. Since the alcohol is immiscible with the triglyceride or the fatty acid starting material, a rate of the reaction is limited by the mixing rate of the starting materials. As the transesterification reaction proceeds, two products are formed, the alkyl ester and the glycerol. One phase includes the alkyl ester and the other phase includes the glycerol. The liquid phases are allowed sufficient time to settle and separate before additional processing steps are conducted to purify the alkyl ester from the glycerol. If large amounts of glycerol remain in the alkyl ester phase, the quality of the biodiesel fuel is diminished. Similarly, the quality of the glycerol is diminished if the glycerol is contaminated with the alkyl ester. Significant problems arise in separating the two phases because the phases are separated by gravity and the separation produces large amounts of waste water, which adds to the cost and complexity of the process. The process is also long and requires numerous hours to process each batch of the triglyceride or the fatty acid starting material.
To improve the reaction rate, supercritical fluids have been used to dissolve the starting materials into a single critical phase. As disclosed in International Application WO 00/05327 to Ginosar et al., the alcohol and the triglyceride (or the fatty acid) are solvated in a critical fluid, which provides a single phase in which the reaction occurs. However, in order for the reaction to proceed, the temperature and pressure conditions must be sufficiently high to maintain the fluid in the critical phase. Therefore, the reaction requires elevated temperatures, pressures, and/or solvent flows.
It would be desirable to increase the rate of the transesterification reaction or esterification reaction and to improve the separation of the alkyl ester from other products of the reaction.